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copied!<p>One trivial way would be to use <code>imageData</code> but that would be painfully slow. It's an option, but not a good one for a game engine.</p> <p>Another way would be to think of the ambient light and the light-source as if they were one path. That would make it very easy to do:</p> <p><a href="http://jsfiddle.net/HADky/" rel="noreferrer">http://jsfiddle.net/HADky/</a></p> <p>Or see it with an image behind: <a href="http://jsfiddle.net/HADky/10/" rel="noreferrer">http://jsfiddle.net/HADky/10/</a></p> <p>The thing you're taking advantage of here is the fact that any intersection of a path on canvas is always only unioned and never compounded. So you're using a single gradient brush to draw the whole thing.</p> <p>But it gets a bit trickier than that if there's more than one light-source. I'm not too sure how to cover that in an efficient way, especially if you plan for two light-sources to intersect.</p> <p>What you should probably do instead is devise an alpha channel instead of this overlay thing, but I can't currently think of a good way to get it to work. I'll revisit this if I think of anything else.</p> <hr> <p><strong>EDIT:</strong> Hey! So I've done a bit of thinking and came up with a good solution.</p> <p>What you need to do is draw a sort of alpha channel, where the dark spots mark the places where you want light to be. So if you had three light sources it would look like this:</p> <p><img src="https://i.stack.imgur.com/MLfs7.png" alt="enter image description here"></p> <p>Then you want to set the fill style to your ambient color and set the globalCompositeOperation to xor and xor the whole thing.</p> <pre><code>ctx.fillStyle = amb; ctx.globalCompositeOperation = 'xor'; ctx.fillRect(0,0,500,500); </code></pre> <p>That will leave you with the "opposite" image except the transparent parts will be correctly ambient:</p> <p><img src="https://i.stack.imgur.com/695Vv.png" alt="enter image description here"></p> <p>Here's a working example of the code:</p> <p><a href="http://jsfiddle.net/a2Age/" rel="noreferrer">http://jsfiddle.net/a2Age/</a></p> <p>Extra optimization: You can actually achieve the effect without using any paths at all, by simply drawing the exact same radial gradients onto rects instead of circular paths:</p> <p><a href="http://jsfiddle.net/a2Age/2/" rel="noreferrer">http://jsfiddle.net/a2Age/2/</a> Hope that helps!</p>

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